1. Field of the Invention
This invention relates to the control of a high intensity discharge lamp. Specifically it addresses the design of a ballast to minimize the flicker caused by acoustic resonance instability.
2. Discussion of Related Art
Driving high-pressure arc discharge lamps can produce standing pressure waves inside the arc discharge tube, which may cause the discharge to become chaotic. The chaotic discharge can result in an annoying flicker of the light output. There exists a certain range of frequencies where the lamp can be driven without causing arc instability. Electronic ballasts can be designed to operate within this acoustic-resonance-free window. However, as the lamp ages and for other reasons not yet understood, acoustic resonance instability can occur irregularly within this window.
Conventionally, the operating frequencies at which acoustic resonance is likely can be computed, and the ballast is designed to avoid operation at these frequencies. Such designs, however, are highly dependent upon the physical characteristics of the particular lamp, and similarly dependent upon the change of these characteristics as the lamp ages or is exposed to differing environmental conditions.
Canadian Patent Application 2160864 discloses the use of an initial test phase wherein the frequency of operation is varied within a range, and the lamp is monitored for quiet and stable operation at each frequency. After this test phase is completed, the lamp is set to operate within the window formed by the largest number of contiguous frequencies which provided quiet and stable operation during the test phase. The test phase is initiated with each startup of the lamp, and thereby adjusts for unstable operations caused by the aging of the lamp. During the test phase, the lamp may be operated at a frequency at which acoustic resonance is produced, which may induce visually disturbing flicker.
Recognizing that acoustic resonance causes a change in the lamp voltage and/or current, U.S. Pat. 5,569,984 discloses the sampling of a characteristic of the lamp, such as its conductance, about a span of frequencies, and then setting the operating frequency to that frequency which exhibits the least amount of deviation in that characteristic. This sampling is performed both during initial startup, as well as when the lamp is in a steady state condition.
In each of these implementations, the proper operating frequency is determined by purposely operating the lamp at a variety of frequencies, some of which may induce acoustic resonance, and then selecting the proper operating frequency from among those which did not cause acoustic resonance during the sampling, or test, period.